Americans may add as many as two years to the nation’s life expectancy if they can stand up more often and watch fewer hours of television, a study found. While studies have shown that too much time sitting and watching TV are bad for a person’s health, today’s research in the journal BMJ Open is the first to show how reducing sedentary behaviors may increase life expectancy in the U.S., now about 78 years, said Peter Katzmarzyk, the lead author.